This invention relates to an improvement in a speech recognition system.
Extensive applications of speech recognition systems responsive to an input oral or speech signal can be found in data input means for computers as well as control information input means for various machines. Recently, speech recognition systems have come to be put into practical use to feed routing information into automatic package sorting machines or various inspection data into computers at automobile factories or elsewhere as described by Thomas B. Martin in the article entitled "Practical Applications of Voice Input to Machines" published in the Proceedings of the IEEE, Vol. 64, No. 4, April 1976, pp. 487 to 501.
Such a conventional speech recognition system determines time segments, in which an oral input signal is present (referred to as speech signal durations hereunder), by converting the input signal into an electrical signal with the use of a microphone and by monitoring the amplitude of said electrical signal so that a speech recognition may be achieved through spectrum analysis and recognition of the electrical signal given in said speech signal duration.
No problem will be caused if such speech recognition system is used under a quiet environment, whereas it is vulnerable to misrecognition under a noisy environment. Namely, when intensive noise is generated while the oral input signal is being given, the speech spectrum superimposed with a noise spectrum of substantial amplitude may be distorted, and the input signal may be recognized differently from what it is intended to be. In this case, if the erroneous recognition is conveyed to the machine to be controlled (for example, a package sorting machine), the machine will cause a faulty operation. To avoid such a disadvantage, those speech recognition systems are so designed as to reject the erroneous recognition results whose apparent accuracy fails to satisfy a certain standard (hereinafter called the rejection standard). More in detail, in the conventional speech recognition system, such a rejection standard is kept unchanged once it is preset. If this standard is too loose, the above-mentioned misrecognition frequently occurs under a noisy environment, or if it is too strict, the recognition result of a slightly inarticulate speech signal may be rejected even under a quiet environment. On the other hand, it is possible to select in advance and fix the optimum rejection standard if the intensity of noise is constant, but the noise generating conditions will change every moment under an actual environment as affected by ambient noise due to a start or stop operation of a motor, a typewriting operation, or a telephone call.